Various devices have been used to determine the degree of tilt of a surface. For example, conventional bubble levels are widely utilized in construction and many other applications. However, the use of such devices is limited because most bubble levels are operable about only a single axis. These instruments are also generally ineffective for measuring very small angles of inclination. Moreover, when such a level is tilted the bubble often tends to stick in its glass tube due to surface anomalies in the tube. As a result, inaccurate measurements may be provided.
In a more sophisticated device such as a capacitor pick-off or an electrolytic bubble level, the bubble is typically included in an electrical circuit. As the apparatus is tilted the bubble shifts position thereby altering various electrical characteristics, e.g., capacitance, resistance, in the circuit. One such device employs a chamber filled with an electrolytic fluid that contains the bubble. A flexure extends from the chamber to a support base and a number of electrodes extend into the fluid. As the flexure tilts one or more of the electrodes are exposed by the bubble and the resistance in the circuit increases. Servo-drive mechanisms are then activated to restore the flexure to its original condition and determine the degree of tilt of the surface in question. Inasmuch as it requires a flexure and servo-drive mechanism, this apparatus is rather complex and expensive. It also exhibits the bubble-sticking problem and to reduce this difficulty the device typically employs an expensive gold plating.
An alternative instrument employs a pendulous mirror mounted by a knife-edge structure on a sapphire bearing surface. The lower end of the pendulum is immersed in a damping fluid. An autocollimator mounted on the housing of the apparatus detects changes in the relative position of the mirror as the housing and autocollimator are tilted. This device is also relatively expensive and intricate and must be calibrated regularly. It requires an autocollimator and its bearing surface often exhibits stickiness which hinders movement of the pendulum.
Accelerometers may be employed to determine surface inclination. However, such devices have wider frequency responses and tend to detect even very high frequency vibrations. As a result, they may sense even minor, transitory and brief disturbances which are not indicative of surface inclination. This makes them inconvenient for use as tilt meters which typically respond to a relatively slow displacement. Even if damped to eliminate high-frequency vibrations, accelerometers exhibit a number of disadvantages when used in this manner. Typically, they are active devices which require an additional active element such as a servo mechanism or coil to restore or correct a tilted component. These active elements indirectly measure the degree of displacement and therefore the surface inclination. In addition to their added structure, complexity and cost, accelerometers are often unsatisfactory because they employ magnetic components and, as a result, tend to be ineffective when used in a strong magnetic field.